ABSTRACT This paper describes an experimental study of heat transfer in oscillating flow inside a cylindrical tube. Profiles of temperature are taken inside the wall and in the fluid from an instrumented test rig, in different conditions of oscillating flow. Profiles obtained allow the observation of the wall effect on heat transfer. A method using the inverse heat conduction principle allows the characterization of local heat transfers at the fluid–solid interface. Finally, a comparison between global and local approaches of heat transfer shows the difficulty of defining a dimensionless heat flux density to model local heat transfer in oscillating flow.

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Heat exchangers are important components of thermoacoustic devices. In oscillatory flow conditions, the flow and temperature fields around the heat exchangers can be quite complex, and may significantly affect heat transfer behaviour. As a result, one cannot directly apply the heat transfer correlations for steady flows to the design of heat exchangers for oscillatory flows. The fundamental knowledge of heat transfer in oscillatory flows, however, is still not well-established. The aim of the current work is to develop experimental apparatus and measurement techniques for the study of heat transfer in oscillatory flows. The heat transferred between two heat exchangers forming a couple was measured over a range of testing conditions. Three couples of finned-tube heat exchangers with different fin spacing were selected for the experiment. The main parameters considered were fin spacing, fin length, thermal penetration depth and gas displacement amplitude. Their effects on the heat exchanger performance were studied. The results were summarised and analysed in terms of heat transfer rate and dimensionless heat transfer coefficient: Colburn-j factor. In order to obtain the gas side heat transfer coefficient in oscillatory flows, the water side heat transfer coefficient is required. Thus, an experimental apparatus for unidirectional steady test was also developed and a calculation method to evaluate the heat transfer coefficient was demonstrated. The uncertainties associated with the measurement of heat transfer rate were also considered.

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Disturbing a single-phase laminar internal convective flow with a particular pulsating flow frequency alters the thermal and hydrodynamic boundary layer, thus affecting the inter-particle momentum and energy exchange. Due to this externally imposed flow disturbance, augmentation in the heat transfer may be expected. Obviously, parameters like pulsating flow frequency vis-à-vis viscous time scales and the imposed pulsating amplitude will play an important role. Conclusions from reported literature on this and related problems are rather incoherent. Lack of experimental data, especially in micro-/mini internal convective flow situations, with imposed flow pulsations, motivates this study. Non-intrusive infra-red thermography has been utilized to scrutinize heat transfer augmentation during single-phase laminar pulsating flow in a square mini-channel of cross-section 3 mm × 3 mm, electrically heated from one side by a thin SS strip heater (70 μm, negligible thermal inertia); all the other three sides of the channel are insulated. The study is done at different pulsating flow frequencies of 0.05 Hz, 1.00 Hz and 3.00 Hz (Wo = 0.8, 3.4 and 5.9, respectively). These values are chosen because pulsatile velocity profiles exhibit different characteristics for Wo > 1 (annular effect, i.e., peak velocity near the channel walls) and Wo < 1 (conventional parabolic profile). Local streamwise heat transfer coefficient has been determined using the time averaged spatial IR thermograms of the heater surface and the local fluid temperature, linearly interpolated from measured value of inlet and outlet bulk mean mixing temperature. It is observed that for measured frequency range, the overall enhancement in the heat transfer is not attractive for laminar pulsating flow in comparison to steady flow with same time-averaged flow Reynolds number. The change is either marginal or highly limited, primarily occurring in the developing length of the channel. Thus, the results suggest that heat transfer enhancement due to periodic pulsating flow is questionable, and at best, rather limited.

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